1. Field of the Invention
This invention relates to an antenna system capable of transmitting electromagnetic radiation and, more particularly, to microwave transmitters associated with an antenna which allows the user to select antenna polarization.
2. Background Information
It is known in the radio frequency (RF) field that feeding two linearly polarized antenna elements with quadrature signals yields circular polarization. It is also known that operating on the relative phase of each of these signals changes the sense of the circular polarization. This technique is generally limited to radar and jamming applications where the amount of cross-polarization signal is less important. However, in the field of communications, dual polarization is often used to increase the number of independent signals that can share a common spectrum. Therefore, for communication applications, it becomes critical to minimize the level of the cross-polarized signal to facilitate the maximum number of independent signals.
The conventional method for generating high quality switchable dual polarization is to use an antenna with two input ports, one for each of the desired polarizations. An RF switch is then used to connect to the appropriate input port of the antenna, thus allowing selection of the desired polarization. Also, in order to meet the output power requirements of many implementations, several amplifiers must be used. As a result, one or more couplers are required for combining the outputs of multiple amplifiers. One must also keep in mind that the RF switch which is selected must be capable of switching the full output power of these amplifiers. In light of this, it is desirable to eliminate the need for high power switching and couplers, reducing the amplification required to overcome the losses inherent in these devices. Finally, it is desirable to alternately utilize a quadrature coupler for recombining quadrature signals, allowing application to linearly polarized antenna systems.